This invention relates to an induction air handling unit, and in particular a nozzle design for an induction air handling unit.
When a gas is discharged from a duct or when it flows through a restriction in a duct, the momentum of the jet is dissipated through mixing with the downstream surroundings. A by-product of this process is the generation of noise which is radiated to the surroundings. It is well established that the sound power which is generated increases at approximately the eighth power of the jet velocity. The efficiency with which it is radiated into the far field of the surroundings depends strongly on both the rate and the scale of the mixing. For a turbulent jet of given mass flow emerging from an orifice of given cross-sectional area, the total sound power radiated decreases as the rate of mixing of the jet with the surroundings increases.
For many years it has been known that the theory of aerodynamic noise generation from turbulent shear flows advanced by Sir James Lighthill, and published in the Proceedings of The Royal Society of London, (On sound generated aerodynamically, Proc. Roy. Soc. A211, p.564, 1952--see also: Waves in fluids, Cambridge University Press, 1978) is incomplete in that it fails to yield reliable predictions of the noise generated by jets at low Mach number. The incompleteness of the Lighthill theoretical model is understandable when it is realised that it was devised before the discovery by G. L. Brown and A. Roshko (Journal of Fluid Mechanics, 64, 775-816, 1974) that the growth of the mixing layer, from which most of the noise emanates, is not continuous but is dominated by the formation of seemingly deterministic vortex-like structures of a scale which is comparable to or larger than the local thickness of the shear layer, and their non-deterministic, intermittent growth by a succession of "amalgamations" between themselves which appear to be little influenced by the turbulent shear layer which they wrap into their structures like jam into a Swiss roll. The magnitude of the disturbances in the flow is many times that which occurs in a simple turbulent shear flow and hence it is reasonable to surmise that these Brown-Roshko vortices may be generating much of the noise. Recent research by Professor N. W. M. Ko and his student Mr R. C. K. Leung at The University of Hong Kong, which has been submitted for publication in the Journal of Sound and Vibration, has advanced a new concept of the noise generation mechanism based on this surmise. The new concept derives from measurements of the processes by which successive Brown-Roshko vortex structures in the shear layer between the jet and the surroundings "pair" together causing the large scale folding, mixing and the intermittent expansion of the jet cross-section. Ko and Leung have found that if a vortex "ring" formed in the mixing layer at the edge of an axisymmetric jet is to "pair" with its predecessor, it must be accelerated rapidly by the pressure field of the leading vortex until it passes through the "eye" of that leading vortex. It is then rapidly retarded and the two vortices merge to become a single larger vortex. The process can be likened to an "extrusion" of the trailing vortex through the eye of the leading vortex. During this "extrusion" process the rates of change of the acceleration of the trailing vortex are very large. In the subject of Mechanics the rate of change of acceleration is known as the "jerk". It is expressed mathematically as the third derivative of distance with respect to time. (It will be recalled that the first derivative of distance with respect to time is the velocity and the second derivative is the acceleration. In solid mechanics it is well known that "jerk" is frequently accompanied by noise; the collision of two marbles and the tapping of a pencil on a table are typical examples). Associated with this extrusion process is a distortion of the shape of the trailing vortex from a nearly circular doughnut shape into a scalloped shape bearing similarities to the nozzle described herein.
The present invention relates to the design of nozzles which stimulate the rapid mixing of jets which discharge into either free or confined surroundings. Of particular interest is the discharge of conditioned air through nozzles for the purposes of cooling or heating a space. An example of this application which involves discharge into "free" surroundings is the "spot cooling" provided for passengers in aircraft. In this application the background noise level is such that the low level of noise achieved by the subject nozzle is of secondary importance compared with the feature of enhanced mixing with the air in the aircraft cabin. A high rate of mixing will allow the same degree of cooling with a smaller quantity of air supplied at a lower temperature than is used in present practice. This would both save energy and produce a more comfortable local cooling around the head and face of passengers without there being an aggressive draught. When applied to the main air conditioning outlets in the aircraft cabin the enhanced mixing and lower temperature of the supply air would also reduce the volume of air needed to cool the cabin when the aircraft is on the ground in a hot climate, or a higher temperature could be used to heat the aircraft when in flight or on the ground in a cold climate.
Other applications of a similar nature are the "spot" cooling or heating of the driver of, for example, an agricultural tractor, a fork lift truck, an excavator, a Load-Haul-Dump vehicle above or below ground, and a crane, among many others. The driver and passengers or crew of a bus, a heavy transport vehicle, a rail vehicle, an armoured military vehicle, an automobile or other transport vehicle would also benefit from the use of the present nozzles in the conditioned air supply system. Again, in these examples the background noise level is high and the low noise characteristic of the subject nozzle is of secondary interest relative to its ability to achieve rapid mixing of the primary air jet with the surroundings. This ability allows the conditioned air to be admitted close to the occupant of the vehicle without producing an undesirable level of draught. Other applications of the nozzles, in all of which excessive draught is undesirable and in most of which a low level of noise is desirable, are exemplified by "spot" cooling or heating of personal work spaces within a factory, an office, a space craft or a submarine, the cooling of electronic components or equipment, the cooling of processes and mechanisms.
An induction air conditioning system relies on the discharge through nozzles as jets of a first or primary stream of cooled and dehumidified, or heated and if necessary humidified air into a confined space within an induction air conditioning unit before discharging to the conditioned space, herein referred to as the room. One boundary of the confined space within the induction unit takes the form of heat exchange means through which a secondary stream of air, originating from the room, is drawn to replace the quantity of air from within said confined space which is entrained into the primary air jet or jets. This occurs naturally because the entrainment by the primary air jet or jets causes the static pressure in the confined space to be reduced below the pressure surrounding the induction unit. The psychrometric state of the secondary stream of air may be changed as it passes through the heat exchange means. The mixture of the primary air and the secondary air streams is then discharged into said conditioned space to provide the required cooling or heating and to provide ventilation.
In such induction air conditioning systems the primary air stream usually consists of air from outside the building often, but not necessarily, mixed with a proportion of air returned from the conditioned space. This primary air is treated in one or more primary air treatment plants before it is ducted to the induction units so that, after having been mixed with the induced secondary air stream within the induction air conditioning units, it is at the temperature and humidity ratio necessary to offset the sensible and latent heat loads in the conditioned space. When used in conjunction with the invention described in Australian Patent 662336 entitled Air conditioning for humid climates, which is now commonly referred to as the High Driving Potential, or HDP system, the primary air can be deeply cooled and dehumidified before being mixed with the entrained secondary air from the room. The efficient mixing produced by the jets from the multi-lobe nozzles will ensure that the air mixture which reaches the occupants is at the desired temperature and moisture content. The most common application of induction air conditioning systems is to condition the air in the space bounded by the building perimeter walls and an often imaginary line some 3 to 6 meters in from said perimeter walls on each level of the building.
The space so defined is referred to as the perimeter zone. A perimeter zone may be physically defined by partitioned offices or may be open space which merges with the interior zone of the building. A conventional air conditioning system usually feeds the whole of the treated air, at modest pressure, from a plant room or air supply shaft through ducts mounted above the ceiling, and thence to ceiling mounted supply air registers distributed throughout the space. Such supply air ducts, because they convey the whole of the conditioned supply air at low pressure, necessarily have relatively large cross-sections. In combination with the depth of the structural beams associated with the floor slab of the next level of the building, they set the required height of the ceiling space and therefore have a determining influence on the required slab-to-slab spacing. In many cities or parts of cities a height restriction is placed on buildings. Thus the size of the air conditioning ducts in the ceiling has a major influence on the number of levels or floors in the building, and hence on the rentable floor space.
Because perimeter induction units carry only primary treated air and do so at relatively high pressure, they are much smaller in cross-section than are the conventional supply air ducts. In one example in the city of Adelaide in South Australia, the use of an induction system to air condition the perimeter zones of the building allowed thirteen levels to be built within a height restriction appropriate to a conventional twelve story building.
The treated primary air streams in a perimeter induction system supply to the perimeter zone at least that quantity of pre-treated outdoor air which is required, by regulation or by best practice, to ventilate the zone. A common criterion used by designers is to require the primary air to offset heat which is transmitted through the perimeter walls and windows which bound the perimeter zone. The heat exchange means within the perimeter induction units which treat the induced secondary air are designed to offset all other loads which originate within the conditioned space of the perimeter zone including people, electrically powered devices, and lighting.
In addition to the abovementioned advantage of requiring less ceiling space than conventional air conditioning systems, induction systems require smaller and hence less expensive and less intrusive ducts to supply air from the primary air plant to each level of the building and to the conditioned space on each level. They do not require separate plant rooms which intrude into the potentially rentable space. Thus in terms of invested capital they are less expensive both to purchase and to install than are conventional air conditioning systems and they increase the proportion of the building which is counted as rentable space. Hence the return on investment can be larger than for conventionally serviced buildings. These advantages have in the past caused induction air conditioning systems to be preferred by many building owners and developers. Many such systems have been installed in buildings in many countries since the second world war.
Despite the apparent economic advantages of the system from the viewpoint of building developers, and from the viewpoint of building owners who pass on to their tenants the operating costs of the air conditioning, induction air conditioning systems have proved to be less than well received by tenants.
Because the induction units are located within the conditioned space, tenants are exposed to the noise generated by the primary air jets as they entrain the secondary air which is induced from the conditioned space to flow into the units through the heat exchange means. This noise has frequently been cause for complaint by tenants. Research by the Trane Company Inc (J. B. Custer, "The economics and marketing of tenant comfort", Proc. AIRAHFAIR-88, Sydney, AIRAH, 1988) has shown that discontent with the air conditioning, expressed through complaints about the operating cost, "staleness" of the air in the conditioned space, or the noise level, is one of the most common reasons reported by tenants for terminating a building lease. That research also showed that from the building owner's viewpoint, the cost of losing a tenant, finding and installing another is typically equivalent to approximately six months rental income from the leased space. Such losses can rapidly erode the advantage of the lower capital cost per unit of rentable area in the building.
A more important problem which magnifies tenant discontent is that in warmer climates the cooling capacity of that quantity of treated primary air which is required for ventilation is insufficient to offset the transmission load to the perimeter zone. Furthermore the quantity of secondary air which can be induced to flow through the secondary air heat exchange means by the jets supplying only ventilation air as the treated primary air is almost always inadequate to offset the internally generated load within the perimeter zone. Hence it has been necessary to increase the quantity of treated primary air both to offset the transmission load and to induce sufficient secondary air to flow through the secondary air heat exchange means to offset the loads generated within the perimeter zone. The increase of treated primary air is effected by increasing the pressure at which said primary air is supplied to the nozzles.
This increases the velocity at which the primary air is discharged from the nozzles. As indicated above, the noise generated by a jet is approximately proportional to the eighth power of its velocity.
Thus the increased cooling capacity is obtained at the direct cost of treating a greater quantity of hot and/or humid outdoor air. Another potential direct cost of the increased primary air pressure is tenant discontent due to the further increase in the noise radiated from the induction units into the conditioned space. For thirty years after the second world war the cost of energy remained low and operating costs were of small importance, thus the direct cost could be tolerated. That period was also one of rapid economic growth; office space was in short supply and hence tenants were unwilling to terminate a lease. Thus the inconvenience of the noise was tolerated. The very different economic climate of the 1990's with its surfeit of office space in many countries, higher cost of energy and growing concern about global warming has changed the situation substantially. Tenants find relocation both economically and environmentally attractive; owners find that while rental margins remain low and buildings are not fully occupied, operating costs are a serious concern.
To improve the occupational health of existing buildings equipped with induction air conditioning systems, and to improve their profitability for the owners of such buildings, it is an object of this invention to specify a nozzle and a means of profiling one or more of the boundaries of the confined space within existing induction air conditioning units in such manner that the interaction of the two will overcome the abovementioned problems. Similar principles are applicable also for new designs of induction air conditioning system and for the design of zone control boxes for conventional Variable Air Volume (VAV) systems.
It is a further object of the invention to specify a nozzle which can reduce the volume of noise generated at the outlet from a duct or at a change in the cross-section of a duct. More specifically, it is an object of the invention to increase the rate at which a primary air stream can induce a secondary air stream to flow through secondary air heat exchange means so to increase the effectiveness of induction air conditioning units and allow the velocity and hence the supply pressure of the primary air stream, and hence the noise generated by the jets, all to be reduced. As stated above, the noise generated by a jet is approximately proportional to the eighth power of the jet velocity. Hence it is apparent that a reduction in jet velocity can have a dramatic effect on the noise radiated from said induction air conditioning units or from VAV control boxes.